The hydrogen behavior in nanocrystalline W (ncW) samples with grain size
of 5 and 10 nm is studied using Molecular Dynamics (MD) with a bond
order potential (BOP) for the W-H system. The dependence of the hydrogen
diffusion coefficient on grain size (5 and 10 nm) and hydrogen
concentration (0.1 at.% < H < 10.0 at.%) is calculated. These data
show that in all cases the hydrogen diffusion coefficient is lower for
ncW than for coarse-grained samples. Trapping energies of grain
boundaries are estimated and a broad distribution roughly centered at
the vacancy trapping energy is found. Hydrogen diffusion results are
interpreted within the trapping model by Kirchheim for nanocrystalline
materials. The H-H interaction is evaluated and the possible formation
of H-2 is disregarded for the conditions in these simulations. Hydrogen
segregation and trapping in grain boundaries for ncW is discussed,
including extrapolations for micron-sized polycrystals. (C) 2014
Elsevier B.V. All rights reserved.